For the predisable hook, the disable code should occur before detachingthe poll func, and for the postenable hook, the poll func should beattached before the enable code.

The lmp9100 was attaching a poll function but never detaching it via anyIIO disable hook.

This change adds the detaching of the poll function, and moves/renameslmp91000_buffer_preenable() function to lmp91000_buffer_postenable().The idea is to make it more symmetrical, so that when theiio_triggered_buffer_{predisable,postenable} functions get removed, it'seasier to see.